Transducers generally convert electrical signals to mechanical signals or vibrations, and/or mechanical signals or vibrations to electrical signals. Acoustic transducers, in particular, convert electrical signals to acoustic signals (sound waves) and convert received acoustic waves to electrical signals via inverse and direct piezoelectric effect. Acoustic transducers generally include acoustic resonators, such as surface acoustic wave (SAW) resonators and bulk acoustic wave (BAW) resonators, and may be used in a wide variety of electronic applications, such as cellular telephones, personal digital assistants (PDAs), electronic gaming devices, laptop computers and other portable communications devices. For example, BAW resonators include thin film bulk acoustic resonators (FBARs), which may be used for electrical filters and voltage transformers. Generally, an acoustic resonator has a layer of piezoelectric material between two conductive plates (electrodes), which may form a thin membrane.
FBAR devices, in particular, generate longitudinal acoustic waves and lateral acoustic waves when stimulated by an applied time-varying electric field, as well as higher order harmonic mixing products. The longitudinal acoustic wave, usually called a piston mode, is electrically excited by a vertical electric field between electrode plates and has a form of laterally uniform motion with the boundaries of motion determined by an overlap of top and bottom electrodes and the piezoelectric material. Lateral acoustic waves, usually called lateral modes, are excited at the edges of the piston mode motion and facilitate continuity of appropriate mechanical displacements and stresses between electrically excited and non-excited regions. In general, lateral modes are specific forms of motion supported by a mechanical stack and have both longitudinal and shear components. The lateral modes can either propagate freely (so called propagating modes) or exponentially decay (so called evanescent and complex modes) from the point of excitation. These modes can be excited both by a lateral mechanical discontinuity (for example, at an interface between a frame and a membrane, or at the edge of a top or bottom electrode) or by electrical discontinuity (for example, at an edge of a top electrode where the electric field is terminated abruptly). The lateral modes and the higher order harmonic mixing products generally have a deleterious impact on functionality.
In certain configurations, a frame may be provided along one or more sides of an FBAR to mitigate acoustic losses at the boundaries by minimizing scattering of electrically excited piston mode at the top electrode edges and by improving confinement of mechanical motion to the active region of the FBAR (the region of overlap of the top electrode, the piezoelectric layer, and the bottom electrode). In general, frames are made of added (or removed) thin layers of material along the perimeter of the resonator device with the purpose of lowering (increasing) the cutoff frequency in that region with respect to the main membrane. This in turn minimizes the amplitude of the electrically excited piston mode and the resulting scattering at top electrode edges above (or below) the cut-off frequency of a membrane. Frames also create an acoustic impedance mismatch that enables suppression of the amplitudes of propagating and/or evanescent modes (whichever exist in the frequency range of interest) mechanically excited at the membrane/frame boundary, thus further minimizing acoustic energy leakage to the outside of the active region. However, in addition to improved acoustic energy confinement, as well as further improvements in FBAR quality factor Q due to the better acoustic energy confinement, simplified design and implementation of frames are needed. In particular, in some applications, frames placed above the piezoelectric layer are not effective in suppressing modes confined to the bottom part of the stack. Thus, approaches allowing for construction of planarized frames below piezoelectric layers that would facilitate growth of good-quality planar layers above the frame regions are needed.